As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semi-conductor wafers. The basis for these techniques is the notion that a subject may be examined by analyzing the reflected energy that results when a probe beam is directed at the subject. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in magnitude are analyzed. Scatterometry refers to determining properties of the subject from reflectometry and ellipsometry measurements, when the subject scatters or diffracts the probe beam. Such a subject, for example, is the developed photoresist mask applied to a wafer in order to etch a pattern into one of the layers on the wafer. The pattern, for example, can be isolated or densely arrayed lines or holes.
Techniques of this type may be used to analyze a wide range of attributes. This includes film properties such as thickness, crystallinity, composition and refractive index. Typically, measurements of this type are made using reflectometry or ellipsometry as described more fully in U.S. Pat. Nos. 5,910,842 and 5,798,837 both of which are incorporated in this document by reference. Critical dimensions (CD) including spacing, width, height and profile of lines or holes are another class of attributes that may be analyzed. Measurements of this type may be obtained using monochromatic scatterometry as described in U.S. Pat. Nos. 4,710,642 and 5,164,790 (McNeil). Another approach is to use broadband light to perform multiple wavelength spectroscopic reflectometry measurements. Examples of this approach are found in U.S. Pat. No. 5,607,800 (Ziger); U.S. Pat. No. 5,867,276 (McNeil) and U.S. Pat. No. 5,963,329 (Conrad). Still other tools utilize spectroscopic ellipsometric measurement. Examples of such tools can be found in U.S. Pat. No. 5,739,909 (Blayo) and U.S. Pat. No. 6,483,580 (Xu). Each of these patents and publications are incorporated herein by reference
An ellipsometer measures the ratio of the amplitudes and the phase difference of reflection from the subject in two orthogonal polarizations. Let rP and rS denote the complex reflection coefficients of the subject for the polarizations where the electric field is in the plane of incidence (S) and perpendicular to the plane of incidence (P), respectively. An ellipsometer measures |rP|/|rS| and angle(rP)-angle(rS). A spectroscopic ellipsometer measures these quantities for each wavelength, over a range of wavelengths. Ellipsometers do not measure |rP|, |rS|, angle(rP), angle(rS) separately. Therefore, ellipsometers do not require absolute amplitude or path length calibrations and they are not sensitive to drifts in those quantities. On the other hand, an ellipsometer owes its precision to not attempting to measure difficult to measure quantities, which nevertheless contain information about the subject. One objective of embodiments described herein is to measure |rP|, |rS|; and angle(rP), angle(rS) separately, up to an arbitrary path length.
For interferometry, an optical beam is subdivided into two portions before reaching the subject. The first portion is reflected by the subject. The second portion is recombined with the first portion after the first portion has been reflected. The recombination creates interference between the first and second beam portions. The interference may be modulated by changing the optical path traveled by one of the two beam portions either by changing the distance or the refractive index.
As described in U.S. Pat. No. 5,923,423 (Sawatari) interferometry has been used to scan un-patterned wafers for particles and defects.
White-light interferometry combined with imaging, also called coherence probe microscopy, may also be used to characterize critical dimensions, such as line spacing, line width, wall depth, and wall profiles. Applications of this nature are described in U.S. Pat. No. 4,818,110 (Davidson) and U.S. Pat. No. 5,112,129 (Davidson). Each of these patents and publications are incorporated herein by reference. In each of these applications, an image of the subject under test is constructed. The image shows the magnitude of the energy reflected by the subject modified by a pattern of interference. Multiple images are captured as the phase of the interference is modulated. The dimension that is perpendicular to the wafer is probed by interferometry to locate the top and bottom boundaries of three-dimensional features. The dimensions in the plane of the wafer are obtained from the image; therefore, such measurements are limited by the resolution of the optical imaging system.
Broadband coherence interferometry is used to map nanometer-scale surface topography of wafers (B. Bhushan, J. C. Wyant, and Chris Koliopoulos, “Measurement of surface topography of magnetic tapes by Mirau interferometry,” Applied Optics, Vol. 24, No. 10, 1489–1497, 15 May 1985; J. F. Valley, C. L. Koliopoulos, S. Tang, Proc. SPIE Vol. 4449, p. 160–168, Optical Metrology Roadmap for the Semiconductor, Optical, and DataStorage Industries II, A. Duparre, B. Sing, Eds., SPIE Press, Bellingham, Wash., December 2001) These techniques measure nanometer-scale of topography in the direction that is perpendicular to the wafer but they are limited by the resolution of the microscope, on the order of 0.5 micrometers for visible-light microscopy, in the plane of the wafer.
Currently, scatterometry measures dimensions in the plane of the wafer with a precision that is an order of magnitude higher compared to microscopy techniques that use a similar wavelength range. An objective of embodiments described herein is to further advance the performance of scatterometry by measuring individual amplitudes and phases of the p- and s-polarizations up to an arbitrary path length. Absolute phases of the s- and p-polarizations are not measurable because such a measurement would be sensitive to minute (10 nanometer scale) changes in the optical path length. Thermal expansion and vibration preclude absolute phase measurements. An objective of embodiments described herein is to measure the phase of the reflections in the p- and s-polarizations, for a range of wavelengths, up to an uncertain path length that is constant during the acquisition of at least one data set.